ABSTRACT The 9/11 World Trade Center terrorist attack was a massive disaster, resulting in long-term physical and psychological trauma to the responders, in particular PTSD and lower respiratory symptoms (LRS), with 10-20% of responders experiencing symptoms consistent with the diagnosis of PTSD a decade later. Our group found that PTSD in WTC responders is closely linked to their respiratory diseases, and PTSD mediated the relationship between WTC exposures and LRS. Genetic vulnerability and gene-environment interactions have been implicated in the etiology of PTSD. In our pilot study of a discovery cohort of N=195 WTC responders, we found significant differences in gene expression between responders with versus without a current WTC-PTSD diagnosis, using whole genome transcriptome analysis. Expression of relevant genes in remitted PTSD cases was intermediary between current cases and non-cases. Importantly, previously implicated genes (FKBP5, NR3C1) were differentially expressed along with several interleukin (IL4, IL2), insulin receptor, B-cell, lymphocytes and PTEN signaling pathways that are linked to inflammatory diseases and to cancers. We tested the resulting expression composite in an independent replication sample of N=87 responders and achieved an AUC of .734. The composite also showed a robust linear association with a self- reported measure of PTSD symptom severity, the Posttraumatic Stress Disorder Checklist (PCL) (Spearman r=0.422). These preliminary findings support the potential importance of gene expression research to identify biomarkers of PTSD. The proposed study builds on this exciting pilot work by evaluating association between changes in gene expression with changes in PTSD symptom severity and LRS across an 18-month period. Using our banked blood samples, we will conduct state-of-the-art high-throughput RNA-sequencing (RNA-Seq) to generate transcriptome profiles from whole blood at an average sequencing depth of 50M reads per sample per time point (for each of the two time points). In Aim 1 of this study, we will investigate the association between changes in gene expression (gene, isoform and splice variant levels) and changes in PTSD symptoms across an 18-month period at gene as well the genetic pathways implicated by these changes; and identify the Gene Expression Signature (GES) associated with change in PTSD symptoms. In Aim 2, we will evaluate whether the GES is associated with change in each PTSD symptom dimension (re-experiencing, avoidance, numbing, hyperarousal). We expect to find both common and distinct genes/pathways regulating the change in each PTSD symptom dimension. In Aim 3, we will test the directionality of prospective associations between the GES score and PTSD symptom severity, i.e., whether the GES at the first time point predicts PTSD severity 18 months later, and vice versa. In Aim 4, we will evaluate whether the change in GES is associated with change in LRS. We hypothesize that any association observed between the GES and LRS will be mediated by change in PTSD symptom severity, indicating shared biological mechanisms underpinning PTSD-LRS comorbidity. This will be the first longitudinal study to identify the gene and isoform expression signatures associated with change in PTSD symptoms overall, as well as each of four PTSD symptom dimensions and comorbid LRS. An in-depth understanding of the biological processes underpinning PTSD, and identification of its GES carries clinical significance by identifying processes that maintain PTSD and can be targeted for intervention, and will inform treatment development for WTC responders as well as other trauma-exposed populations.